Pattern reading apparatus

ABSTRACT

A pattern reading apparatus for reading a pattern from a reflective object or a transparent object. The pattern reading apparatus includes a minute-area light source, an objective lens system, an imaging lens, and an imaging element. The objective lens system causes the illumination light beam from the light source to be incident on the object and converges the light beam reflected from or transmitted through the object. The imaging lens forms an image of the object using only a scattered component of light which has been reflected from or which has passed through the object. The imaging element is disposed at a position where the image of the pattern is imaged for reading the pattern. In particular, light that forms an image of the light source is prevented from reaching the imaging element. In the case of a reflective object, ghosting light due to a reflection from a surface of the objective lens is prevented from reaching the imaging element.

BACKGROUND OF THE INVENTION

The present invention relates to a pattern reading apparatus for readinga pattern formed on a surface of a silicon wafer or the like, and morespecifically, to a pattern reading apparatus for reading a patternformed on a reflective or transparent surface.

In manufacturing semiconductor products, semiconductor layers areapplied to a semiconductor substrate, such as a silicon wafer or thelike, by vapor deposition and then design patterns are formed byphoto-lithography processes, etching processes, and the like. Ingeneral, a serial number is applied to the silicon wafer by laseretching so that the silicon wafer can be tracked during the patternforming processes based on the serial number. Conventionally, the serialnumber on the silicon wafer is discriminated by a worker visuallyexamining the wafer.

However, since the silicon wafer is mirror finished, for a worker toread the serial number, the wafer must be viewed obliquely while holdingit to the light, or by some similar method. Further, since the qualityof the pattern may deteriorate as the silicon wafer is subjected toprocesses such as etching, vapor deposition and the like, it isparticularly difficult to discriminate the serial number of the siliconwafer after a number of such processes.

Conventionally, two types of pattern reading devices have been known: areflective-type reading device, and a transmission-type reading device.The former is used for reading a pattern formed on a reflective surface,and the latter is used for reading a pattern formed on atransmission-type surface.

In an example of the reflective-type reading device, light emitted by alight source is incident, through a lens, to a surface on which thepattern is formed, and an image of the pattern is formed by an imaginglens on a screen or the like. In this case, a portion of the lightincident to the lens is reflected on a surface of the lens to createghosting light, which reaches the screen and reduces contrast of theimage of the pattern. Further, the specular reflection from the surfacehaving the pattern formed thereon may be incident on the screen makingit more difficult to observe the image of the pattern.

As an example of the transmission-type reading device, a known devicehas a Fourier transformation lens, that is used for reading a patternformed on a light-transmission-type object by subjecting the pattern toa predetermined filter processing. In these optical systems, the lightbeam from a point light source passes through a first lens and isincident on an object as a parallel light beam. After passing throughthe object, the light beam is converged by a second lens and caused topass through a spatial filter disposed at the back focal point of thesecond lens. When an imaging lens, having the front focal point set tothe position of the filter, is disposed behind the filter, an objectimage, which is affected by the function of the filter, is formed at theback focal point of the imaging lens.

For example, to output an emphasized image of a pattern formed on anobject surface, a high-pass filter may be used as the spatial filter toshade the paraxial rays which correspond to the image of the point lightsource. Further, an imaging element may be disposed at the imagingposition to capture and process the image for further processing ordisplaying on a display unit.

In the above conventional filtering optical system, however, when anobjective lens (first lens) has spherical aberration such as, forexample, a spherical single lens or when coma and curvature of fieldarise because a light beam is obliquely incident on the objective lens,there is a problem in that the light beam which forms the image of apoint light source does not converge to a point but scatters over alarger area such that a large shading region must be provided toproperly execute filtering. Thus, a quantity of light used to form theimage is lowered.

In a pattern reading apparatus using the above conventional filteringoptical system, since the magnification of a pattern image having beenformed cannot be changed, the pattern image cannot be optically enlargedor reduced. That is, since an object surface is disposed to the focalpoint of an objective lens in the conventional optical system, the lightbeam emitted from the objective lens is made afocal. Thus, even if theimaging lens is moved, magnification cannot be changed. To change themagnification, the imaging lens must be composed of a group of aplurality of lenses.

Further, a pattern reading apparatus using the above conventionalfiltering optical system cannot be easily used when the object to beread is intended to function as a prism (i.e., has a wedge shape or thelike) for deflecting a light beam. In this case, the image of the pointlight source will not be shaded by a spatial filter because the imagewill be formed at a position outside of the axis. Thus, a component oflight other than the scattered reflected component will be incident onan imaging lens and a desired filtered output image cannot be output. Asimilar problem also may arise when a reflection surface is tilted atthe time a pattern is read by this type of apparatus.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a pattern readingapparatus capable of forming a high-contrast image of an indistinctpattern such as a serial number or the like formed on a mirror surfacesuch as a silicon wafer, and in particular, capable of even reading apattern which has deteriorated because of processing such as etching,vapor evaporation, and the like.

A second object of the present invention is to provide a pattern readingapparatus capable of reading a pattern image even if a portion of anillumination light beam is reflected at the lens surface of an objectivelens or even if an object surface is somewhat irregular.

A third object of the present invention is to provide a pattern readingapparatus, which includes a filtering optical system, capable of shadingthe light beam that forms the image of a point light source withoutlowering the quantity of light of the pattern image substantially, evenif the image of the point light source is expanded due to sphericalaberration, coma, and curvature of field of an objective lens.

A fourth object of the present invention is to provide a pattern readingapparatus using a filtering optical system in which the magnification ofa pattern image may be changed using a simple structure.

A fifth object of the present invention is to provide a pattern readingapparatus, using a filtering optical system, with which an image of apoint light source and a shading region of a spatial filter can be madeto coincide, even if an object has a function of a prism or even if areflection type object has a tilted reflection surface.

According to an aspect of the present invention, there is provided, apattern reading apparatus including a minute-area light source, anobjective lens, an imaging lens, and an imaging element. The objectivelens causes the illumination light beam from the light source to beincident on a reflection surface having a pattern formed thereon as anobject to be read and converges the light beam reflected from thereflection surface. The imaging lens is for imaging an image of thepattern by a scattered reflected component, which has passed through theobjective lens, of the reflected light beam. The imaging element isdisposed at a position where the image of the pattern is imaged forreading the pattern. The light source is optically conjugate with acenter of curvature of the surface of the object to be read through theobjective lens.

According to another aspect of the present invention, there is provided,a pattern reading apparatus including illumination means forilluminating a reflection surface having a pattern formed thereon as anobject to be read by a parallel light beam and detection means fordetecting an image by imaging a scattered reflected component ofillumination reflected from the reflection surface.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus, an objective lens, a spatialfilter, and an imaging lens. The imaging lens forms the image of thepattern using the light beam that passes through the spatial filter. Thepattern reading apparatus includes a minute-area light source forcausing an illumination light beam to be incident on an object surfacehaving a pattern formed thereon as an object to be read. The objectivelens converges a light beam carrying the information of the pattern. Thespatial filter is disposed at a position where a size of an image of thelight source formed by the objective lens is smaller than a size of theimage at a paraxial image point. The spatial filter has a shading regionfor shading a portion of the light beam that forms an image of the lightsource from the light beam.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus including a minute-area lightsource, an objective lens for converging a light beam having theinformation of the pattern, a spatial filter, and an imaging lens forforming the image of the pattern by the light beam having passed throughthe spatial filter. The minute-area light source causes an illuminationlight beam to be incident on an object surface having a pattern formedthereon as an object to be read. The spatial filter is disposed nearerto the objective lens than the paraxial image point of the image of thelight source. The spatial filter also has a shading region for shadingthe light beam for forming the image of the light source which iscontained in the light beam having passed through the objective lens.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus including a minute-area lightsource, an objective lens, a spatial filter, an imaging lens, and animaging element disposed at the imaging position of the pattern imagefor reading the pattern. The objective lens causes the illuminationlight beam from the minute-area light source to be incident on an objectsurface having a pattern formed thereon as an object to be read andconverges the light beam reflected at the object surface. The spatialfilter is disposed nearer to the objective lens than the paraxial imagepoint of the light source formed through the objective lens forcapturing the scattered reflected component which is contained in thereflected light beam having passed through the objective lens. Theimaging lens forms an image of the pattern by the component havingpassed through the spatial filter.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus using a Fourier conversion opticalsystem composed of a first lens, an object surface to be read, a secondlens, a spatial filter, and an imaging surface which are disposed alongthe traveling direction of the light beam from a light source. Thespatial filter is disposed nearer to the second lens than the back focalpoint of the second lens.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus for causing a light beam emittedfrom a light source to be incident on an object surface. The objectsurface has a pattern formed thereon as an object to be read through anobjective lens. The pattern reading apparatus is also for converging thelight beam reflected at the object surface through the objective lens aswell as reading the image of the pattern by forming the image by animaging lens. Also provided is a tilt mechanism for supporting theobjective lens such that the objective lens is rotatable about arotation axis which is perpendicular to the optical axis of theobjective lens.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus including a minute-area lightsource, an objective lens, a spatial filter, an imaging lens, an imagingelement, and a tilt mechanism. The objective lens causes theillumination light beam from the light source to be incident on anobject surface having a pattern formed thereon as an object to be readand converges the light beam reflected at the object surface. Thespatial filter captures the scattered reflected component which iscontained in the reflected light beam having passed through theobjective lens. The imaging lens images the image of the pattern by thecomponent having passed through the spatial filter. The imaging elementis disposed at the imaging position of the pattern image for reading thepattern. The tilt mechanism supports the objective lens to allow turningabout a turning axis which is perpendicular to the optical axis of theobjective lens.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus for causing the illumination lightbeam emitted from a minute-area light source to be incident on an objectsurface through a first lens. The object surface has a pattern formedthereon as an object to be read. The pattern reading apparatus has asecond lens that converges a light beam having the information of thepattern and causes the converging light beam to be incident on animaging lens. The pattern reading apparatus is for forming the image ofthe pattern by the imaging lens and reading the formed image. A tiltmechanism is provided for supporting the second lens to allow turningabout a turning axis which is perpendicular to the optical axis of thesecond lens.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus for causing the illumination lightbeam emitted from a minute-area light source to be incident on an objectsurface. The object surface has a pattern formed thereon as an object tobe read. The pattern reading apparatus has an objective lens forconverging a light beam having the pattern information. The patternreading apparatus also causes the converging light beam to be incidenton an imaging lens, which forms the image of the pattern, and reads theimage. The objective lens is disposed such that the light beamoriginating from a point of the object surface and emitted from theobjective lens is changed to a non-parallel light beam. The imaging lensand an imaging surface are made movable along the optical axis directionof the imaging lens in order to change magnification.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus including a minute-area lightsource, an objective lens, a spatial filter, and an imaging element. Theobjective lens causes the illumination light beam from the light sourceto be incident on an object surface, having a pattern formed thereon asan object to be read, and converges the light beam reflected at theobject surface. The spatial filter is for capturing a scatteredreflected component which is contained in the reflected light beamhaving passed through the objective lens. The imaging element isdisposed at the imaging position of the pattern image for reading thepattern. The imaging lens and the imaging element are movable along theoptical axis direction of the imaging lens in order to changemagnification.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus for causing the illumination lightbeam emitted from a minute-area light source to be incident on an objectsurface, having a pattern formed thereon as an object to be read. Thepattern reading apparatus also has an objective lens that converges alight beam having the pattern information and causes the converged lightsource to be incident on an imaging lens. The imaging lens forms theimage of the pattern. The pattern reading apparatus is also for readingthe image and includes an adjustment mechanism for adjusting theposition of the light source in a plane which is perpendicular to theprincipal beam of the illumination light beam.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus for causing the illumination lightbeam emitted from a minute-area light source to be incident on an objectsurface having a pattern formed thereon as an object to be read. Anobjective lens converges a light beam having the pattern information.The light beams that pass through the objective lens are incident on animaging lens through a spatial filter. The imaging lens forms the imageof the pattern. The image is also read. The spatial filter is a filterhaving a shading region for shading paraxial rays. The apparatusincludes an adjustment mechanism for adjusting the relative positionalrelationship between the position of the image of the light sourceformed by the objective lens and the shading region of the spatialfilter in the plane which crosses the optical axis of the imaging lens.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus including a minute-area lightsource disposed to cause an illumination light beam to be obliquelyincident on an approximately flat object surface having a pattern formedthereon as an object to be read at a predetermined incident angle. Thepattern reading apparatus also includes an objective lens for converginga light beam having the information of the pattern, a spatial filterhaving a shading region for shading the portion of the reflected lightbeam from the object surface which has passed through the spatial filterand forms the image of the light source, and an imaging element. Theimaging element is for reading the image of the pattern formed by thelight beam having passed through the spatial filter. The line extendingfrom the principal plane of a lens interposed between the object surfaceand an imaging surface and having an imaging action, the line extendingfrom the imaging surface and the line extending from the object surfacecross each other on an approximately straight line.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus including a minute-area lightsource for illuminating an object surface having a pattern formedthereon as object to be read. The pattern reading apparatus alsoincludes an objective lens for converging a light beam having thepattern information, a spatial filter and a shift mechanism. The spatialfilter has a shading region for shading the light beam, which forms theimage of the light source, of the light beam having passed through theobjective lens. The shift mechanism is for supporting the objective lensso as to allow parallel movement in a direction approximatelyperpendicular to the optical axis of the objective lens. The image ofthe pattern formed by the component having passed through the spatialfilter is read.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus including a minute-area lightsource disposed such that an illumination light beam is caused to beincident on an object surface having a pattern formed thereon as aobject to be read without passing through a lens. The pattern readingapparatus also includes an objective lens for converging a light beamhaving the pattern information, a spatial filter, and an imagingelement. The spatial filter has a shading region for shading theportion, which forms the image of the light source, of the light beamhaving passed through the objective lens. The imaging element is forreading the image of the pattern formed by the light beam having passedthrough the spatial filter.

According to yet another aspect of the present invention, there isprovided, a pattern reading apparatus including an objective lensdisposed in confrontation with an object surface as a reflection surfacehaving a pattern formed thereon as an object to be read. The patternreading apparatus includes a minute-area light source disposed at aposition which is conjugate with the center of curvature of the objectsurface through the objective lens for illuminating the object surfacethrough the objective lens. The pattern reading apparatus also includesan imaging lens and an imaging element. The imaging lens is disposedfarther from the object surface than the light source with the opticalaxis thereof in coincidence with the objective lens. The imaging elementis for reading the image of the pattern which is reflected at the objectsurface and formed through the objective lens and the imaging lens.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 shows an optical system of a pattern reading apparatus accordingto a first embodiment;

FIG. 2 is a plan view showing an example of a spatial filter;

FIG. 3 is a plan view showing another example of a spatial filter;

FIG. 4 shows a specific arrangement of an optical system according tothe first embodiment;

FIG. 5 shows a first modification of the first embodiment;

FIG. 6 shows a second modification of the first embodiment;

FIG. 7 shows a third modification of the first embodiment;

FIG. 8 shows a fourth modification of the first embodiment;

FIG. 9 shows a fifth modification of the first embodiment;

FIG. 10(A) shows an optical system of a pattern reading apparatusaccording to a second embodiment;

FIGS. 10(B) and 10(C) show a modification of the optical system of FIG.10(A) for making magnification adjustable;

FIGS. 11(A) and 11(B) show an optical system of a pattern readingapparatus according to a third embodiment;

FIGS. 12(A) and 12(B) show an optical system of a pattern readingapparatus according to a fourth embodiment;

FIGS. 13(A), 13(B), and 13(C) show an optical system of a patternreading apparatus according to a fifth embodiment;

FIG. 14 shows an optical system of a pattern reading apparatus accordingto a sixth embodiment;

FIG. 15 shows the optical system of FIG. 14 in a developed form;

FIG. 16 shows a specific arrangement of an optical system according tothe sixth embodiment;

FIGS. 17(A) through 17(H) are spot diagrams showing the size of an imageof a light source calculated based on the specific arrangement of FIG.16;

FIG. 18 is a front view of a specific mechanical arrangement of apattern reading apparatus including the optical system of FIG. 14;

FIG. 19 is a side view showing the apparatus of FIG. 18;

FIG. 20 illustrates the movement loci of an imaging lens and an imagingelement for adjusting magnification;

FIG. 21 illustrates an alternative arrangement for adjusting a pinholeunit;

FIGS. 22(A) and 22(B) illustrate an alternative arrangement foradjusting a spatial filter;

FIG. 23 shows the arrangement of FIG. 22 as mounted;

FIG. 24 shows a modification of an optical system according to the sixthembodiment;

FIGS. 25(A) and 25(B) show an optical system of a pattern readingapparatus according to a seventh embodiment;

FIG. 26 shows an optical system of a pattern reading apparatus accordingto an eighth embodiment;

FIG. 27 is a plan view showing the arrangement of a shift mechanism formoving an objective lens;

FIG. 28 is a side view of the shift mechanism of FIG. 27;

FIG. 29 is a plan view showing an alternative arrangement of the shiftmechanism of FIG. 27;

FIG. 30(A) shows an optical system of a pattern reading apparatusaccording to a ninth embodiment;

FIG. 30(B) shows a modification of the optical system of FIG. 30(A);

FIG. 31 shows an optical system of a pattern reading apparatus accordingto a tenth embodiment;

FIG. 32 shows a modification of the optical system of FIG. 31;

FIG. 33 shows an optical system of a pattern reading apparatus accordingto an eleventh embodiment;

FIG. 34 shows a modification of the optical system of FIG. 33;

FIG. 35 shows an optical system of a pattern reading apparatus accordingto a twelfth embodiment; and

FIG. 36 shows a modification of the optical system of FIG. 35.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of a pattern reading apparatus according to the presentinvention will be described below.

First Embodiment

FIG. 1 is a schematic view showing the arrangement of the patternreading apparatus according to a first embodiment. As shown in FIG. 1, asilicon wafer OR has a reflective surface 1 a on which a pattern (inthis case, a serial number) is formed by laser etching. Further, thepattern reading apparatus includes an optical system composed of anillumination unit 10, an objective lens 20 and a detection unit 30. Theobjective lens 20 is disposed so that the optical axis Ax thereof isperpendicular to the reflective surface 1 a. The illumination unit 10and the detection unit 30 are disposed approximately symmetrically withrespect to the optical axis Ax on opposite sides thereof.

The illumination unit 10 includes a lamp 11 such as a halogen lamp, orthe like, and a pinhole plate 12 in which a pinhole 12 a is formed topermit a portion of the light beam emitted from the light source to passtherethrough to form a minute-area light source (the minute-area lightsource will also be referred to as a point light source). A diffusionplate 13 is interposed between the lamp 11 and the pinhole plate 12 toeliminate any effect due to an image of a filament of the lamp 11.

The detection unit 30 includes a spatial filter 31, an imaging lens 32,and an imaging element 33, such as a CCD image sensor, or the like. Inthe embodiment shown in FIG. 1, the detection unit 30 is disposed on aline extending in a direction in which light from the minute-area lightsource will be specularly reflected from the surface 1 a.

The objective lens 20 is designed such that the minute-area light sourceis conjugate with the center of curvature of an object surface to beread. In this embodiment, since the surface 1 a is formed as a plane,the pinhole 12 a is disposed at a front focal point position (i.e., aposition on a plane which is perpendicular to the optical axis Ax of theobjective lens 20, and which includes a front focal point of theobjective lens 20) of the objective lens 20. A light beam emitted fromthe minute-area light source becomes a parallel light beam after passingthrough the objective lens 20 and obliquely illuminates the surface 1 aof the silicon wafer OR. The parallel light beam is scatteringly(diffusely) reflected at edges of the pattern and specularly reflectedat portions other than the edges.

The reflected light beam passes through the objective lens 20 again, andbecomes a converging light beam directed toward the detection unit 30.The spatial filter 31 is disposed at a position where it is conjugatewith the minute-area light source through the objective lens 20, thatis, at a back focal point position of the objective lens 20 in theoptical path between the imaging lens 32 and the objective lens 20(i.e., a position on a plane which is perpendicular to the optical axisAx of the objective lens 20, and which includes a back focal point ofthe objective lens 20). As a result, at the spatial filter 31, aspecularly reflected component of the light beam reflected from thesurface 1 a is converged to a beam spot approximately the same size asthe pinhole 12 a. As shown in FIGS. 2 and 3, the spatial filter 31 isprovided with a shading portion for shading the specularly reflectedcomponent. Specifically, as shown in, for example, FIG. 2, the spatialfilter 31 has a shading portion 31 a for covering the central portion ofthe pupil of the imaging lens 32 which corresponds to a range on whichthe specularly reflected light beam is incident and the right half ofthe pupil of the imaging lens 32. In another example, the spatial filter31 has, as shown in FIG. 3, a shading portion 31 b for covering only thecentral portion of the pupil of the imaging lens 32 which corresponds toa range on which the specularly reflected light beam is incident. In theabove examples, the spatial filter 31 may have a transparent glass plateas shown by broken lines, and the shading portion 31 a or 31 b is formedby a coating or the like.

The diffusely reflected component of the light beam reflected at thesurface 1 a (see FIG. 1), which passes through the spatial filter 31, isincident on the imaging lens 32. A power and a position of the imaginglens 32 is designed such that the surface 1 a of the silicon wafer ORand the imaging element 33 are conjugate with respect to the imaginglens 32, and, thus, the image of the pattern is formed on the imagingelement 33 by the scatteringly (diffusely) reflected component. Theimaging element 33 converts the formed image of the pattern into anelectric signal and outputs the signal to an image processing apparatus(not shown). The image processing apparatus may display the image of thepattern on a display screen based on the input image signal and/oranalyze the content of the pattern using a character recognitionalgorithm.

In the example in FIG. 1, since the detection unit 30 is disposed on theline extending in the direction in which the specularly reflectedcomponent is reflected from the surface 1 a, if the spatial filter 31 isnot provided, the specularly reflected component will be incident on theimaging lens 32. Since the specularly reflected component does notinclude information of the pattern and has a strong intensity, if thespecularly reflected component is captured by the imaging element 33,the Signal to Noise (S/N) ratio of the information of the pattern islowered and it is difficult to detect the pattern. To cope with thisproblem, the S/N ratio of the information of the pattern is improved byremoving the specularly reflected component using the spatial filter 31and permitting the imaging element 33 to capture only the diffuselyreflected component so that it is easy to recognize and discriminate thepattern. Because the image formed on the imaging element is mainlyformed of a high frequency component of a spatial frequency of thecapture image, by suppressing the low frequency component thereof, theedge portion of the captured image of the pattern is actuallyemphasized.

The focal length of the imaging lens 32 is determined based on amagnification determined in accordance with the length of the pattern(i.e., the length of the serial number) and the size of the imagingsurface of the imaging element 33. Further, the focal length of theobjective lens 20 is determined based on the distance between thesurface 1 a and the imaging lens 32, where the distance between thesurface 1 a and the imaging lens 32 is set according to the focal lengthof the imaging lens 32 and the magnification.

FIG. 4 illustrates a design example of the pattern reading opticalsystem of a first embodiment. In this example, the imaging lens 32 has afocal length of 28 mm and the objective lens 20 has a focal length of250 mm. Further, the distance al from the optical axis Ax of theobjective lens 20 to the pinhole 12 a is about 60 mm, the distance b1from the pinhole 12 a to the surface 1 a of the silicon wafer OR isabout 300 mm and the distance c1 from the objective lens 20 to thesurface 1 a is about 50 mm. Assuming that the length of the pattern is 2cm, the image of the pattern is about 1.96 mm long on the imagingelement 33. Thus, the imaging element 33 may be, for example, ½ inch inlength.

FIGS. 5 to 7 show modifications of the optical system according to thefirst embodiment. In the modifications, the illumination unit 10, theimaging lens 32, and the imaging element 33 of the detection unit 30 arethe same as those of the first embodiment shown in FIG. 1.

In a first modification shown in FIG. 5, the imaging lens 32 is disposedin a position at which the specularly reflected component will not beincident thereon. In this way, the spatial filter 31 is not required.More particularly, in the example of FIG. 5, the imaging lens 32 isdisposed at a position which is farther from the optical axis Ax of theobjective lens 20 than in FIG. 1. Accordingly, only the diffuselyreflected component is incident on the imaging lens 32, and an image ofthe pattern, in which the edge portion is emphasized, is formed on theimaging element 33 without the spatial filter 31.

In a second modification, shown in FIG. 6, the illumination unit 10 isdisposed on the optical axis Ax of the objective lens 20 such that anillumination light beam is incident on the surface 1 a of the siliconwafer OR at a right angle (i.e., along the optical axis Ax). In thismodification, a beam splitter 40 is disposed in the optical path betweenthe pinhole plate 12 and the objective lens 20 to separate the opticalpath of the illumination light beam emitted from the illumination unit10 from the optical path of the reflected light beam from the surface 1a.

The illumination light beam from the pinhole 12 a passes through thebeam splitter 40 and the objective lens 20 to become a parallel lightbeam (also parallel with the optical axis Ax) that illuminates thesurface 1 a. The reflected light beam from the surface 1 a passesthrough the objective lens 20 again and becomes a converging light beam,a part of which is reflected at the beam splitter 40 toward the spatialfilter 31. A position of the spatial filter 31 is conjugate with theminute-area light source, similar to the first embodiment, and shadesthe specularly reflected component of the reflected light beam. Thediffusely reflected component passes through the spatial filter 31 andthe imaging lens 32 to form an image of the pattern on the imagingelement 33.

In a third modification, shown in FIG. 7, an objective lens is composedof an illumination lens (a first lens) 21 through which an illuminationlight beam passes and an objective lens (a second lens) 22 through whichthe reflected light beam from the surface 1 a of the silicon wafer ORpasses. These lenses 21, 22 are disposed such that the optical axes Ax1,Ax2 thereof cross each other at the surface 1 a of the silicon wafer OR.The other arrangement and operation of the third modification are thesame as those of the first embodiment. It should be noted that in thethird modification, the minute-area light source is located at a focalpoint of the first lens 21, and the filter 31 is located at a focalpoint of the second lens 22.

FIGS. 8 and 9 show further modifications of the first embodiment. Thesemodifications have substantially the same structure as that of thesecond modification of FIG. 6. The modification shown in FIG. 8 is usedwhen an object to be read has a convex spherical surface 1 b and themodification shown in FIG. 9 is used when an object to be read has aconcave spherical surface 1 c.

In FIG. 8, an objective lens 20 makes a minute-area light sourceprovided by a pinhole 12 a conjugate with the center of curvature of theobject surface 1 b and causes an illumination light beam to be incidenton the object surface 1 b as a converging light beam that issubstantially perpendicular to the object surface 1 b. The illuminationlight beam is diffusely reflected at edges of the impressed pattern andspecularly reflected at portions other than the edges. These reflectedcomponents then become incident on the objective lens 20 again. Inparticular, the specularly reflected component passes through theobjective lens 20 along the same optical path as the illumination lightbeam.

In FIG. 9, an objective lens 23 makes a minute-area light sourceprovided by a pinhole 12 a conjugate with the center of curvature of theobject surface 1 c and causes an illumination light beam to be incidenton the object surface 1 c as a diverging light beam that issubstantially perpendicular to the object surface 1 c. The illuminationlight beam is diffusely reflected at edges of the impressed pattern andspecularly reflected at portions other than the edges. The reflectedcomponents are then incident on the objective lens 23 again. Inparticular, the specularly reflected component passes through theobjective lens 20 along the same optical path as the illumination lightbeam.

Note, the minute-area light source may also be a light emitting dioderather than the combination of the halogen lamp 11 and the pinhole plate12 used in the above and following embodiments and modifications.Because the light emitted by the light emitting diode is concentrated ata central portion, the light emitting diode may be suitably used as aminute-area light source that is near to a point light source.Alternatively, a combination of a lamp and an optical fiber may also beused to realize the minute-area light source. That is, the lamp and anincident end of the optical fiber may be located at a separatedposition, and the other end of the optical fiber can be used as theminute-area light source.

As described above, according to the first embodiment, because an imageis formed using only the diffusely reflected component of the reflectedlight beam from an object (i.e., the specularly reflected component isprevented from reaching the imaging device), a pattern, such as a serialnumber, or the like, formed on a reflective surface, such as a siliconwafer, can be easily read. Therefore, the pattern can be easily andaccurately recognized by displaying the pattern or using characterrecognition techniques to decode the pattern. In particular, even if apattern has deteriorated through processes such as etching, vapordeposition, and the like, it can be easily read.

Second Embodiment

FIGS. 10(A), (B) and (C) show optical systems for a pattern readingapparatus according to a second embodiment and modifications thereof.The second embodiment is an example of a filtering optical system fordetecting a pattern contained in a light-transmission-type object OT. Alight beam emitted from a lamp (not shown) passes through a pinholeplate 12 to form a minute-area light source, and the light emitted fromthe minute-area light source is incident on the object OT through anillumination lens (first lens) 21. The light beam then passes through anobjective lens (second lens) 22, a spatial filter 31, and an imaginglens 32, to form an image of the object OT on an imaging surface 33 a.

The spatial filter 31 has a shading region at a center thereof forshading the portion of the light beam from the minute-area light sourcewhich has not been scattered by the object OT. In the second embodiment,the spatial filter 31 is disposed nearer to the objective lens 22 than aparaxial image point IM of the minute-area light source. The spatialfilter 31 is similar to ones shown in FIG. 2 or FIG. 3.

In FIG. 10(A), the objective lens 22 is a Fourier transformation lens.In this case, the minute-area light source is located at the front focalpoint of the illumination lens 21 such that the object OT is illuminatedby a parallel light beam. In addition, the object OT is located at thefront focal point of the objective lens 22 (the Fourier transformationlens). The back focal point of the objective lens 22 coincides with thefront focal point of the imaging lens 32, and the imaging surface 33 ais located at the focal point of the imaging lens 32.

In the first embodiment, the spatial filter 31 is located at theconjugate position of the minute-light source with respect to theobjective lens 20. In other words, in the first embodiment, the spatialfilter 31 is located at the paraxial image point IM of the minute-arealight source, that is, at the back focal point of the objective lens 20.However, if the objective lens includes aberrations such as sphericalaberration, coma, or curvature of field, the spread of the image of theminute-area light source is not reduced to a minimum at exactly theparaxial image point IM.

Thus, in the second embodiment, the spatial filter 31 is disposed at aposition where the image of the minute-area light source is a minimumsize after taking the effect caused by the spherical aberration of theobjective lens 22 and the effect resulting from the coma and curvatureof field caused by abaxial rays into consideration. With thisarrangement, the shading region may be made smaller than a region whichis located at the paraxial image point IM.

Specifically, the spatial filter 31 is disposed at the position whichsatisfies the condition that the distance L from the final surface ofthe objective lens 22 to the spatial filter 31 is 0.60 fo<L<0.95 fo,wherein fo is the focal length fo of the objective lens 22. Because thesize of the image of the light source at a point within the range of0.60 fo<L<0.95 fo is smaller than the size at the paraxial image pointIM (L=fo), the shading region can be made smaller than that of the firstembodiment. Note, when the above arrangement is applied to an actualoptical system, it is preferable to determine a position where the sizeof the image of the minute-area light source is minimized by tracinglight rays. The spatial filter 31 is then placed at an appropriateposition in accordance with the shape of the image.

FIG. 10(B) shows a modification of the optical system in FIG. 10(A)arranged to permit an adjustment of magnification. In the arrangementshown in FIG. 10(A), since the light beam emitted from a point on theobject OT becomes a parallel light beam after passing through theobjective lens 22, as shown by a dotted line, magnification cannot bechanged by moving the imaging lens 32. In order to allow magnificationto be changed by moving the imaging lens 32, the optical system in FIG.10(B) is arranged such that a distance X from the object OT to anobjective lens 22 is set shorter than the focal length of the objectivelens 22. With this arrangement, a light beam from a point on the objectOT passes through the objective lens 22 to be a non-parallel light beam,and accordingly the magnification can be changed by moving an imaginglens 32 and an imaging surface 33 a.

In particular, it is preferable that the distance X satisfies thecondition 0<X<0.7 fo where the focal length of the objective lens is fo.In this arrangement, because the paraxial image point IM where the imageof the light source is formed will also be closer to the object OT, thespatial filter 31 can also be located nearer to the object OT ascompared with the optical system of FIG. 10(A). With this arrangement,the movable range of the imaging lens 32 is larger, providing a widervariable magnification range.

As a further modification, in FIG. 10(C), the position of a minute-arealight source is located farther from an illumination lens 21 than thefront focal point of the illumination lens 21. With this arrangement,since the illumination light beam emitted from the illumination lens 21is a converging light beam, the position of the image of the minute-arealight source formed through an objective lens 22 is formed nearer to theobject OT, so that the movable range of the imaging lens 32 can befurther increased allowing an even wider variable magnification range.

According to the second embodiment and its modifications, since thespatial filter 31 is disposed at the position where the size of theimage of the minute-area light source formed by the objective lens issmallest, the area of the shading region of the spatial filter may bemade as small as possible, such that a bright image of the emphasizedimage of the pattern can be formed. Further, the magnification of theimage formed on the imaging device can be made variable.

Third Embodiment

FIGS. 11(A) and 11(B) show optical systems of the pattern readingapparatus according to a third embodiment. The third embodiment is anexample of a filtering optical system for detecting a pattern containedin a reflection type object to be detected similar to the firstembodiment.

In FIG. 11(A), a light beam from a lamp (not shown) is incident on apinhole plate 12 to form a minute-area light source. Light from theminute-area light source passes through an objective lens 20 and isobliquely incident on a reflection type object OR. The light beamreflected at the object OR is converged through the objective lens 20,passes through an imaging lens 32, and forms a pattern image of theobject OR on an imaging surface 33 a. In this embodiment, the objectivelens 20 is rotatable about a rotation axis Rx that is perpendicular to aplane of incidence and intersects the optical axis Ax of the objectivelens 20.

A portion of the illumination light beam which is incident on theobjective lens 20 from the pinhole plate 12 is reflected by theobjective lens 20 and may be incident on the imaging lens 32 as aghosting light. If the ghosting light overlaps the pattern image, thecontrast of the image is reduced and accordingly it may be difficult toread the pattern image. Thus, in this embodiment, the direction in whichthe ghosting light is reflected is changed by turning the objective lens20. In particular, because the direction of the ghosting light is verysensitive to the rotation of the objective lens 20, but the direction ofthe transmitted light beam is less sensitive to the rotation of theobjective lens 20, it is possible to change a position of the ghostinglight without substantially changing the position of the pattern image.In this embodiment, the spatial filter is not used. By reducing theghosting light, the contrast of the pattern image can be improved tomake it easier for a user to recognize the pattern image.

FIG. 11(B) shows a modification of the third embodiment, arranged suchthat a light beam from a pinhole plate 12 passes through the objectivelens 20 and is perpendicularly incident on the object OR. A portion ofthe reflected light beam from the object OR passes through the objectivelens 20, is reflected at a beam splitter 40, and is incident on theimaging lens 32. As in the arrangement of FIG. 11(A), the objective lens20 is rotatable about the rotation axis Rx that is perpendicular to aplane of incidence and intersects the optical axis Ax of the objectivelens 20. As a result, it is possible to adjust a position of ghostinglight so that the ghosting light does not overlap the pattern image onthe imaging surface 33 a.

The optical system of the third embodiment may also include a spatialfilter similar to that of the first and second embodiments.

In this case, the effect of turning the objective lens can be used tocontrol the position of the image of the minute-area light source formedon the spatial filter in addition to controlling the direction of theghosting light.

According to the third embodiment, when the reflection on the surface ofthe objective lens prevents observation, and when the reflection surfaceof a reflection type object is tilted, a desired filtered output imagecan be obtained. The filtered output image can be obtained by changingthe position where an image is formed by turning the objective lens apredetermined angle about the turning axis of the objective lens. Theturning axis is perpendicular to the optical axis thereof.

Fourth Embodiment

FIGS. 12(A) and 12(B) show an optical system included in a patternreading apparatus according to a fourth embodiment. This is an exampleof a filtering optical system for detecting a pattern contained in alight-transmission-type object, similar to the second embodiment.

A light beam emitted from a lamp (not shown) passes through a pinholeplate 12 to form a minute-area light source. The light emitted from theminute-area light source passes through an illumination lens 21, and isincident on a light-transmission-type object OT. The light beam passesthrough the object OT, through an objective lens 22, through a spatialfilter 31, and forms an emphasized image of the object OT on an imagingsurface 33 a. The spatial filter 31 has a shading region at the centerthereof for shading the light beam which forms the image of the lightsource. In this embodiment, as in the second embodiment, the spatialfilter 31 is disposed nearer to the objective lens 22 than the paraxialimage point IM of the light source. As in the third embodiment, theobjective lens 22 is rotatable about a rotation axis Rx that isperpendicular to the optical axis thereof as shown in FIG. 12(A).

FIG. 12(A) shows a case in which the surface of the object OT is nothomogeneous. In this case, the shape of the image of the minute-arealight source may be deformed and may not conform with the shape of thepinhole. Thus, there is a possibility that the image of the minute-arealight source will not coincide with the shading region of the spatialfilter 31. By rotating the objective lens 22, the shape of the image ofthe minute-area light source is changed due to a change of coma and theobjective lens 22 may be rotated until the image of the minute-arealight source coincides with the shading region.

FIG. 12(B) shows a case in which the object OT is shaped and functionsas a prism. In this case, the light beam forming the image of theminute-area light source is directed slightly upward from the opticalaxis, and may not be shaded by the shading region of the spatial filter31. To cope with this problem, the objective lens 22 is rotated apredetermined acute angle counterclockwise (as shown in FIG. 12(B))about the rotation axis Rx. The rotation of the objective lens 22adjusts the position of the image of the minute-area light source, sothat the light beam which forms the image of the light source isappropriately shaded by the spatial filter 31.

Fifth Embodiment

FIGS. 13(A), 13(B), and 13(C) show an optical system for a patternreading apparatus according to a fifth embodiment. This is an example ofa filtering optical system for detecting a pattern contained in alight-transmission-type object, similar to the second embodiment.

A light beam emitted from a lamp (not shown) passes through a pinholeplate 12 to form a minute-area light source. The light then passesthrough an illumination lens 21, and is incident on alight-transmission-type object OT. The light beam passes through theobject OT, through an objective lens 22, through a spatial filter 31,and forms an emphasized image of the object OT on an imaging surface 33a. The spatial filter 31 has a shading region at the center thereof forshading the portion of the light beam which forms the image of theminute-area light source. In this embodiment, the spatial filter 31 isdisposed nearer to the objective lens 22 than the paraxial imaging pointIM of the minute-area light source. In the fifth embodiment, the lamp(not shown) and the pinhole plate 12 are movable in a planeperpendicular to an optical axis as shown by an arrow A in FIG. 13(A).Further, the spatial filter 31 is also movable in a plane perpendicularto the optical axis as shown by an arrow B in FIG. 13(A).

In the optical system shown in FIG. 13(A), the pinhole plate 12 islocated at the front focal point of the illumination lens 21 and theobject OT is located at the front focal point of the objective lens 22(the Fourier transformation lens). The back focal point of the objectivelens 22 coincides with the front focal point of the imaging lens 32 andthe imaging surface 33 a is located at the focal point of the imaginglens 32.

FIGS. 13(B) and 13(C) show a case in which the object OT has a prismshape. In this case, since the light beam from the object OT isrefracted upward (in the view of FIGS. 13(A), 13(B), and 13(C)), theportion of the light beam that forms the image of the minute-area lightsource may not be shaded by the shading region of the spatial filter 31.In this case, as shown in FIG. 13(B), the light source and the pinholeplate 12 may be moved a predetermined amount upward to adjust theposition of the image of the minute-area light source such that theportion of the light beam which forms the image of the minute-area lightsource is shaded by the spatial filter 31. In FIG. 13(B), a solid lineindicates the case that a pinhole is located on the optical axis and adot-dash-line indicates the case that the pinhole plate is moved upward.Alternatively, as shown in FIG. 13(C), the spatial filter 31 may bemoved a predetermined amount upward (in the view of FIG. 13(C)) in aplane perpendicular to the optical axis in order to ensure that theposition of the image of the minute-area light source coincides with theshading region of the spatial filter 31.

According to the fifth embodiment, even if an object functions as aprism, the image of the light source can be caused to coincide with theshading region of the spatial filter by adjusting the position of thelight source or the position of the spatial filter. The above-describedprinciple can be applied to a system using the reflection type object.

Sixth Embodiment

FIGS. 14 to 23 show a pattern reading apparatus according to a sixthembodiment. The apparatus of the sixth embodiment is for a reflectiontype object and provides all the features described in the second to thefifth embodiments, that is:

(1) the spatial filter 31 is placed nearer to the objective lens 20 thanthe paraxial image point IM;

(2) the magnification is adjustable;

(3) the objective lens 20 is rotatable about the axis Rx which isperpendicular to a plane of incidence; and

(4) the lamp 11 and the pinhole plate 12 and/or the spatial filter 31are movable in a direction perpendicular to the optical axis of theobjective lens 20.

As shown in a schematic view in FIG. 14, the optical system of theapparatus includes an illumination unit 10, an objective lens 20, and adetection unit 30. The objective lens 20 is disposed such that theoptical axis Ax thereof is perpendicular to the surface 1 a of a siliconwafer OR (i.e., a reflection surface) in a standard position. Theillumination unit 10 and the detection unit 30 are disposedapproximately symmetrically on opposite sides of the optical axis Ax ofthe objective lens 20 in the standard position.

The illumination unit 10 includes a lamp 11, a pinhole plate 12 formedwith a pinhole 12 a to form a minute-area light source, and a diffusionplate 13 provided between the lamp 11 and the pinhole plate 12. Thedetection unit 30 includes a spatial filter 31, an imaging lens 32, andan imaging element 33. In the example of FIG. 14, the detection unit 30is disposed on a line which extends in a direction in which a specularlyreflected component of light from the minute-area light source isreflected from the surface 1 a.

The light beam emitted from the lamp 11 passes through the pinhole 12 a,through the objective lens 20, and is incident on the surface 1 a. Thelight beam is reflected at the surface 1 a, passes through the objectivelens 20 again, and is incident on the spatial filter 31. In thisembodiment, the pinhole 12 a (the minute-area light source) ispositioned at a front focal position of the objective lens 20 (i.e., aposition on a plane which is perpendicular to the optical axis Ax of theobjective lens 20, and which includes a front focal point of theobjective lens 20) such that the light beam emitted from the objectivelens 20 is a parallel light beam and obliquely illuminates the surface 1a of the silicon wafer OR. The illumination light beam is diffuselyreflected at an impressed pattern portion of the surface 1 a andspecularly reflected at portions other than the above.

The light beam reflected at the surface 1 a passes through the objectivelens 20 again, is transformed into a converging light beam directedtoward the detection unit 30 and reaches the spatial filter 31. Thespatial filter 31 is disposed nearer to the objective lens 20 than theparaxial image position of the minute-area light source.

The diffusely reflected component, which has passed through the spatialfilter 31, of the light beam reflected at the surface 1 a is incident onthe imaging lens 32. The imaging lens 32 forms the emphasized image ofthe pattern impressed on the surface 1 a on the imaging element 33 bythe diffusely reflected component. The imaging element 33 converts theinformation of the emphasized image of the pattern into an electricsignal and outputs the signal to an image processing apparatus (notshown).

The objective lens 20 is rotatable about a rotation axis Rx, as shown bythe arrow R in FIG. 14. In this embodiment, the rotation axis Rx isparallel with a line where a plane, which is perpendicular to theprincipal beam Ax1 of the illumination light beam, crosses a plane whichis perpendicular the optical axis Ax2 of the imaging lens 32 (i.e., therotation axis Rx is perpendicular to a plane of incidence and crossesthe optical axis Ax). The objective lens 20 is rotatable with a range ofabout ±45 degrees from the standard position.

If a ghosting light, i.e., a reflection at the surface of the objectivelens 20, is incident on the imaging lens 32 and overlaps the position ofthe pattern image on the imaging element 33, the objective lens 20 isrotated so that the ghosting light does not overlap the pattern image.

When the distribution of the diffusely reflected light beam is uneven onthe surface 1 a, there is a possibility that the shape of the image ofthe light source is changed and displaced from the shading region 31 bof the spatial filter 31. In such a case, the shape of the image of thelight source can be changed by controlling coma by turning the objectivelens 20.

Further, the illumination unit 10 is adjustable in a direction, shown bythe arrow S1, in a plane perpendicular to the optical axis Ax in orderto adjust the position of the image of the minute-area light source withrespect to the shading region 31 b of the spatial filter 31. Stillfurther, the spatial filter 31 is adjustable in a direction, shown bythe arrow S2, in a plane that is perpendicular to the optical axis Ax2of the imaging lens 32.

In this way, if the surface 1 a is tilted, the position where the imageof the light source is formed can be adjusted to coincide with theshading region 31 b by adjusting the position of the illumination unit10 and/or the spatial filter 31.

Still further, the imaging lens 32 and the imaging element 33 arearranged such that they are movable along the optical axis Ax2 of theimaging lens 32, shown by the arrow S3, to change magnification. Inaddition, to permit the magnification to be changed by the movement ofthe imaging lens 32, the distance between the objective lens 20 and thesurface 1 a (object surface) is set to satisfy the condition 0<X<0.7 fo,where fo is the focal length of the objective lens 20. When thiscondition is satisfied, since a light beam emitted from a point on thesurface 1 a is not parallel after passing through the objective lens 20,the magnification can be changed by moving the imaging lens 32 along theoptical axis Ax2.

FIG. 15 shows an expanded optical path of the optical system of FIG. 14.The light beam emitted from the pinhole plate 12, is collimated to be aparallel light beam by the objective lens 20, reflected at the surface 1a (passes therethrough in FIG. 15), is incident on the objective lens 20again, passes through the spatial filter 31 as a converging light beam,passes through the imaging lens 32, and forms an image of the pattern onthe imaging element 33. The optical system in FIG. 14 is fundamentallyequivalent to the optical system of the second embodiment shown in FIG.10(B) except with respect to the incident direction of the light beamand the transmission/reflection characteristics of the object. That is,in the example in FIG. 14, the surface 1 a is nearer to the objectivelens 20 than the focal point thereof and a light beam from a point onthe surface 1 a is incident on the imaging lens 32 not as a parallellight beam but as divergent light.

FIG. 16 shows a design example when it is assumed that the length of apattern to be read is 2 cm and the size of the imaging surface of animaging element is ½ inch across a diagonal. In this example, theimaging lens 32 has a focal length of 50 mm and the objective lens 20has a focal length fo of 220 mm. Further, a distance b from the pinhole12 a to the surface 1 a of the silicon wafer OR is about 270 mm, adistance c from the objective lens 20 to the surface 1 a is about 50 mm,and a distance L from a final surface of the objective lens 20 to thespatial filter 31 is about 190 mm. Therefore, the condition 0.60fo<L<0.95 fo, described above, is approximately 130 mm<L<210 mm in thisexample. Further, the condition 0<X<0.7 fo, also described above, is0<X<154 mm.

FIGS. 17(A) through 17(H) are spot diagrams showing the shape of theimage of a minute-area light source calculated based on the model shownin FIG. 16, that is, the distribution of the light beam from the surface1 a which constitutes the specularly reflected component, at variousdistances DF from the paraxial image point of the minute-area lightsource. The distance DF is 0 at the paraxial image point and a minussign represents a position closer to the objective lens 20. In theexample, since the size of the image of the minute-area light source isminimized about 30 mm or 40 mm closer to the objective lens from theparaxial image point, the disposition of the spatial filter in thisrange permits the specularly reflected component to be shaded by a smallshading region so that a maximum possible quantity of light can be usedto form a bright image of the pattern.

Next, a specific mechanical arrangement of an apparatus including theoptical system shown in FIG. 14 is described with reference to FIGS. 18and 19. Note, as shown in FIG. 18, a coordinate system x, y, z isdefined in which the x-axis is parallel with the optical axis Ax of theobjective lens 20 at the standard position. Further, the principal beamAx1 of an illumination light beam and the optical axis Ax2 of an imaginglens are contained in an x-z plane.

The pattern reading apparatus of the sixth embodiment includes a baseframe 100 on which a silicon wafer (i.e., an object to be inspected) isplaced at a reference position T, shown by a dot-dash-line, and amovable frame 200 which is disposed on the base frame 100, is supportedby bearings 101 so as to slide in the direction y with respect to thebase frame 100.

The movable frame 200 is moved by a frame drive mechanism 210 (shown inFIG. 19). As shown in FIG. 19, the frame drive mechanism 210 includes aball screw 211 which is disposed to a screw support portion 102, securedto the base frame 100 in such a manner that the rotation of the ballscrew 211 can be adjusted, and a threading member 212 which is securedto the horizontal support plate 201 (parallel with a y-z plane) of themovable frame 200. The ball screw 211 includes an operation knob 211 aon an outer side thereof for operation by an inspector and a screwportion 211 b formed on an inner side, i.e., a portion projecting towardthe movable frame 200. The screw portion 211 b of the ball screw 211 isscrewed into a screw hole provided in the threading member 212. Thus,when the ball screw 211 is rotated, the movable frame 200 slides in thedirection y.

The movable frame 200 is provided with a horizontal support plate 201and a tilt mechanism 220 is disposed to the horizontal support plate 201for rotatably supporting the objective lens 20. A column 202 extendsperpendicularly from the horizontal support plate 201 and a verticalsupport plate 203 (parallel with the x-z plane) is secured to the column202. The vertical support plate 203 supports an optical fiber 11 d and apinhole plate 12 which constitute a minute-area light source and animaging unit 320. The imaging unit 320 includes a lens barrel 32Ahousing the imaging lens 32 and a CCD unit 33A housing the imagingelement 33. Optical path holes 100 a, 201 a, 221 a are formed in thehorizontal support plate 201, the base frame 100, and the base plate221, respectively, such that the light beam from the lamp 11 may passthrough to the silicon wafer to allow the reflected light beam from thesilicon wafer to pass to the imaging unit 320.

The tilt mechanism 220 includes the column 202, the base plate 221, anda bearing unit 222 (see FIG. 19). The base plate 221 is disposed betweenthe column 202 and the support member 204 (see FIG. 18). The supportmember 204 extends from the horizontal support plate 201 in parallelwith the column 202. The bearing unit 222 extends below the base plate221. The objective lens 20 is accommodated in a lens frame 223 having arotation shaft 223 a extending in the direction y. The lens frame 223 isrotatably mounted to the bearing unit 222 through the turning shaft 223a. Opposite ends of the turning shaft of the lens frame 223 project fromthe bearing unit 222. A follower pulley 224 is secured to the end of theturning shaft projecting toward the frame drive mechanism 210 and arotary plate 225 of an encoder is secured to the other end thereof.

A lens drive motor 226 is mounted on the base plate 221 of the tiltmechanism 220 and a timing belt 227 is stretched between a drive pulley226 a secured to the rotary shaft of the motor 226 and the followerpulley 224. The encoder is composed of the rotary plate 225 mounted on arotary shaft and a photo interrupter 228 composed of a light emittingelement (not shown) and a light receiving element (not shown) disposedacross the rotary plate 225. The rotary plate 225 has a slit (not shown)formed radially thereon and is adjusted such that when the objectivelens 20 is set at the standard position, a light beam emitted from thelight emitting element of the photointerrupter 228 is detected by thelight receiving element through the slit. As described above, thestandard position of the objective lens 20 in this example is a positionwhere the optical axis Ax of the objective lens 20 is set perpendicularto an ideal object surface (a flat surface).

The lamp 11 is composed of a halogen lamp 11 a, an infrared-ray cutfilter 11 b for reducing a heating component of the converging lightbeam emitted from the halogen lamp 11 a, a negative lens 11 c for makingthe converging light beam an approximately parallel light beam, and anoptical fiber 11 d. A pinhole unit 120 includes the pinhole plate 12, amounting plate portion 122 formed perpendicularly to the pinhole plate12, and a holding unit 121 for holding the exit end of the optical fiber11 d. The pinhole unit 120 is mounted on the vertical support plate 203by bolts 123, 123 through the mounting plate portion 122. Securinggrooves 124, 124 formed in the mounting plate portion 122 extend in aplane which is perpendicular to the axis Ax1 of the illumination lightbeam, so that, by loosening the bolts 123, the unit 120 is easilymovable in a direction perpendicular to the axis Ax1, i.e., closer tothe imaging unit 320 or away from the imaging unit 320.

In this example, the optical fiber 11 d is a commercially availableoptical fiber having a diameter of about 5 mm. Thus, a pinhole 12 a isused reduce the light beam from the optical fiber 11 d to a minute-arealight source, however, if the optical fiber 11 d has a diameter of 1 mmto 2 mm, the pinhole 12 a is not necessary. Further, if the density ofthe light beam emitted from the end surface of the optical fiber 11 d isuneven, it is preferable to provide a diffusion plate (not shown)between the end surface of the fiber 11 d and the pinhole 12 a.

The spatial filter 31 is secured to a filter holder 130 which is thensecured to the vertical support plate 203. The spatial filter 31includes a shading region at the center thereof, similar to that shownin FIG. 3. The spatial filter 31 is arranged at a position which isnearer to the objective lens 20 than the paraxial image point such thatthe size of the image of the light source formed by the objective lens20 is minimized.

The imaging unit 320 is also mounted on the vertical support plate 203.The vertical support plate 203 is formed with a slot 205 in a directionparallel to the optical axis Ax2 of the imaging lens 32. The slot 205includes a first wide step portion 205 a formed on the imaging unit 320side to a depth approximately half a plate thickness and a second narrowstep portion 205 b formed at the center in the width direction of thefirst step portion 205 a passing through the vertical support plate 203from the first step portion 205 a. The imaging unit 320 includes twomounting arms 322 formed thereto that are each inserted through a washer323 provided in the first step portion 205 a of the slot 205 and thensecured by bolts 321 screwed into the ends of each of the arms 322 fromthe opposite side of the vertical support plate 203. The washers 323have a diameter smaller than the first step portion 205 a and largerthan the second step portion 205 b. With the above arrangement, theimaging unit 320 is movable in the direction of the optical axis Ax2 ofthe imaging lens 32. Further, the imaging lens 32 can also be adjustedin the optical axis direction by a lens barrel adjustment mechanism (notshown). Thus, in the embodiment, the magnification can be changed by oneor both of the above two adjustments.

In the apparatus of the sixth embodiment, since the position T isdetermined so that the silicon wafer is located closer to the objectivelens 20 than the focal point thereof, the reflected light beam from apoint on the surface of the silicon wafer is incident on the imaginglens 32 as a divergent light beam. As a result, according to thearrangement of the sixth embodiment, the magnification can be changed bymoving the imaging lens 32 in the optical axis direction. However, whenthe imaging lens 32 is moved to change the magnification, the focusingstate of the pattern to the imaging element 33 is also changed.

Thus, in the embodiment, to change the magnification while maintainingthe focusing state of the pattern, the positions of the imaging lens 32and the imaging element 33 are adjusted, respectively, so that they movealong the loci shown in FIG. 20. In FIG. 20, the magnification graduallyincreases from the upper side to the lower side and the positions of theimaging lens 32 and the imaging element 33 are indicated for the casewhen the surface of the silicon wafer (object surface) is not moved.That is, when the imaging lens 32 and the imaging element 33 are locatedat positions where an arbitrary horizontal straight line crosses therespective locus lines, respectively, a pattern image which is formed onthe imaging element 33 is brought into focus at the relatedmagnification.

When a pattern is to be read, the silicon wafer is placed at thereference position T shown by the dot-dash-line in FIGS. 18 and 19, inparticular, for a pattern of characters, symbols, and the like, thesilicon wafer is positioned so that the lengthwise direction of thepattern coincides with the direction y. After the silicon wafer ispositioned, the halogen lamp 11 a is lit. The light beam emitted fromthe optical fiber 11 d passes through the pinhole 12 a to form anillumination light beam that is obliquely incident on the objective lens20 and is transmitted to the silicon wafer (object surface).

The illumination light beam is reflected at the surface of the siliconwafer, passes through the objective lens 20 again, and is directedtoward the imaging unit 320. The portion of the reflected light beamcorresponding to the image of the light source, that is, the specularlyreflected light beam, is shaded by the shading region 31 b of thespatial filter 31, and other portions of the reflected light beam, thatis, the diffusely reflected light beam, is incident on the imaging unit320. An emphasized image of the pattern is formed on the imaging element33 by the imaging lens 32 and an image signal is read by driving theimaging element 33.

If ghosting light, which is caused by surface reflection at theobjective lens 20, overlaps the pattern image, the tilt of the objectivelens 20 is changed by controlling the lens drive motor 226.

Further, if the surface of the silicon wafer is not flat, for example,if it has a prism shape, the pinhole unit 120 is moved closer to or awayfrom the imaging unit 320 in the plane perpendicular to the principalbeam Ax1 of the illumination light beam so that the image of the lightsource is formed on the shading region of the spatial filter 31.

Note, although the tilt of the silicon wafer itself may also be adjustedto adjust the position of the image of the light source, the apparatusaccording to the present embodiment is arranged for adjusting thepinhole unit 120. In particular, the provision of a tilting mechanismfor adjusting each object to be inspected such that the reflecteddirection of a light beam is accurately controlled would require highsensitivity and a complicated mechanism such that the apparatus would bemore expensive.

FIG. 21 shows an alternative arrangement for adjusting the position ofthe pinhole unit 120.

In this arrangement, a rail member 125 is provided with a guide groove125 a extending in the direction z and is secured to a movable frame(not shown). A pinhole unit 120 a, to which a pinhole plate 12 and theexit end of an optical fiber 11 d are secured, is mounted so as to movein the direction z along the guide groove 125 a.

If this arrangement is combined with the arrangement of FIGS. 18 and 19,the pinhole unit 120 a may be moved in a plane which is perpendicular tothe axis Ax1 of the illumination light beam according to the arrangementshown in FIG. 18 or may be moved in a plane perpendicular to the opticalaxis Ax of the objective lens 20 according to the arrangement shown inFIG. 21.

In a further alternative arrangement, the position of the spatial filter31 may be made adjustable for the purpose of adjusting the relativepositional relationship between the image of the light source and theshading region 31 b of the spatial filter 31. FIGS. 22(A), 22(B), and 23show a mechanism for adjusting the position of the spatial filter 31.FIG. 22(A) is a plan view of a movable frame, FIG. 22(B) is a sectionalview taken along the line B—B of FIG. 22(A), and FIG. 23 is a plan viewshowing the movable frame assembled to fixed rails.

As shown in FIGS. 22(A), 22(B), and 23, the rectangular movable frameincludes two rail members 131 a, 131 a, each having a C-shaped crosssection with the openings thereof facing inward, disposed parallel toeach other and separated by a predetermined distance. Two beam members132 a, 132 b are disposed between the rail members 131 a, 131 a atpositions near opposite ends thereof. The spatial filter 31 is insertedinto the C-shaped openings of the rail members 131 a, 131 b and fixed bypresser screws 133. Guide pins 134 are provided on the movable frame atfour comers thereof and are engaged with two guide grooves 136 a, 136 bformed on fixed rails 135 a, 135 b.

According to the arrangement of FIG. 23, the spatial filter 31 ismovable in the direction Y with respect to the movable frame and themovable frame is further movable in the direction Z by sliding on thefixed rails 135 a, 135 b. Therefore, the position of the spatial filter31 can be adjusted in a Y-Z plane and the position of the shading region31 b of the spatial filter 31 can be adjusted so that the image of thelight source is formed on the shading region 31 b.

FIG. 24 shows a modification of the optical system according to thesixth embodiment. As shown in FIG. 24, the arrangement of theillumination unit 10 and the imaging lens 32 and the imaging element 33of the detection unit 30 are the same as those shown in FIG. 14. In themodification of FIG. 24, the illumination unit 10 is disposed at aposition where an illumination light beam is perpendicularly incident onthe surface 1 a of the silicon wafer OR. That is, a pinhole plate 12having a pinhole 12 a for forming a minute-area light source is disposedon the optical axis Ax of an objective lens 20 which is perpendicular tothe surface 1 a. A beam splitter 40 is disposed in the optical pathbetween the pinhole plate 12 and the objective lens 20 to separate theoptical path of the illumination light beam emitted from theillumination unit 10 from the optical path of the reflected light beamfrom the surface 1 a.

The illumination light beam from the pinhole 12 a passes through thebeam splitter 40 and the objective lens 20 to become a parallel lightbeam (also parallel with the optical axis Ax) that illuminates thesurface 1 a. The reflected light beam from the surface 1 a passesthrough the objective lens 20 again and becomes a converging light beam,a portion of which is reflected at the beam splitter 40 toward thespatial filter 31. The spatial filter 31 is at a position nearer to theobjective lens 20 than a position which is conjugate with theminute-area light source and shades the specularly reflected componentof the reflected light beam from the surface 1 a. The diffuselyreflected component passes through the spatial filter 31 and the imaginglens 32 to form an image of the pattern on the imaging element 33.

Seventh Embodiment

FIGS. 25(A), (B) show an optical system included in a pattern readingapparatus according to a seventh embodiment. The seventh embodiment isan example of a filtering optical system for detecting a patterncontained in a light-transmission-type object OT.

A light beam emitted from a light source (not shown) passes through apinhole plate 12 to form a minute-area light source and is incident onthe object OT through an illumination lens 21. The light beam thenpasses through an objective lens 22, a spatial filter 31, and an imaginglens 32, to form an image of the object OT on an imaging surface 33 a.

The spatial filter 31 has a shading region at the center thereof forshading a portion of the light beam which forms the image of the lightsource and is disposed nearer to the objective lens 22 than the paraxialimaging surface IM of the light source. According to the arrangement,the image of the pattern on the object OT is formed on the image surface33 a only by the scattered component of light from the object OT.

In the optical system shown in FIG. 25(A), the pinhole plate 12 islocated at the front focal point of the illumination lens 21 and theobject OT is illuminated by a parallel light beam. The object OT islocated at the front focal point of the objective lens 22 (the Fouriertransformation lens). The back focal point of the objective lens 22coincides with the front focal point of the imaging lens 32 and theimaging surface 33 a is located at the focal point of the imaging lens32.

FIG. 25(B) shows the case that the object OT has a prism shape, suchthat a light beam is deflected upward (in the view of FIG. 25(B)), thatis, the object OT is formed as a wedge which is thinner at the loweredge (in the view of FIG. 25(B)). In this case, if the objective lens 22is left in the state shown in FIG. 25(A), the image of the light sourcedeviates upward and the portion of the light beam forming the image ofthe light source may not be shaded by the shading region of the spatialfilter 31. To cope with this problem, as shown in FIG. 25(B), theobjective lens 22 is arranged to be movable a predetermined distance ina direction opposite to the direction in which the light beam isrefracted by the wedge, that is, to be moved downward in the view ofFIG. 25(B). The deviation of the image of the light source caused by theprism shape of the object OT can be compensated for by movement of theobjective lens 22. As a result, the portion of the light beam whichforms the image of the light source can be appropriately shaded by thespatial filter 31.

Specifically, in the example in FIG. 25(B), if it is assumed that theobject OT is thinner at the lower side, has an angle (the apex of theprism) of 20 minutes, and a refractive index of 1.5 and the objectivelens 22 has a focal length of 200 mm, the deviation of the image of thelight source caused by the effect of the wedge can be compensated byparallel movement of the objective lens 20 by about 300 μm in adirection which is opposite to the direction in which the object OT isthinner (downward), that is, in the direction in which the light beam isdeflected by the wedge.

Eighth Embodiment

FIG. 26 to FIG. 28 show the arrangement of a pattern reading apparatusaccording to an eighth embodiment. The eighth embodiment is an examplein which the principle of the parallel movement of the objective lens inthe seventh embodiment is applied to an optical system for detecting apattern contained in a light-reflection-type object.

As shown in FIG. 26, the optical system of the apparatus is composed ofan illumination unit 10, an objective lens 20, and a detection unit 30.The objective lens 20 is a bi-convex lens and is disposed such that theoptical axis Ax thereof is perpendicular to the surface 1 a of a siliconwafer OR (reflection surface). The illumination unit 10 and thedetection unit 30 are disposed approximately symmetrically on oppositesides of the optical axis Ax of the objective lens 20. As shown in FIG.26, in this embodiment, the optical axis Ax1 of the illumination unit 10and the optical axis Ax2 of the detection unit 30 cross the optical axisAx of the objective lens at the surface 1 a. The objective lens 20 issupported by a shift mechanism 400 so as to be movable perpendicular tothe optical axis Ax of the objective lens 20 as well as in parallel witha direction X which is parallel with a plane containing the optical axesAx1, Ax2 (which coincides with the paper surface in FIG. 26). The amountof parallel movement M of the objective lens 20 should approximatelysatisfy the following condition:

D/2<M<D/2,

where D is the diameter of the objective lens 20.

The illumination unit 10 includes a lamp 11 such as a halogen lamp, orthe like, and a pinhole plate 12 in which a pinhole 12 a is formed topermit a portion of the light beam emitted from the light source to passtherethrough to form a minute-area light source. A diffusion plate 13 isinterposed between the lamp 11 and the pinhole plate 12 to eliminate anyeffect due to an image of a filament of the lamp 11.

The detection unit 30 includes a spatial filter 31, an imaging lens 32,and an imaging element 33, such as a CCD image sensor, or the like. Inthe embodiment shown in FIG. 26, the detection unit 30 is disposed on aline extending in a direction in which light from the light source willbe specularly reflected from the surface 1 a.

A light beam emitted from the lamp 11 becomes a parallel light beamafter passing through the objective lens 20 and obliquely illuminatesthe surface 1 a of the silicon wafer OR. In particular, the pinhole 12 ais disposed at the front focal position of the objective lens 20 (i.e.,a position on a plane which is perpendicular to the optical axis Ax ofthe objective lens 20, and which includes a front focal point of theobjective lens 20). The parallel light beam is diffusely reflected atedges of the pattern and specularly reflected at portions other than theedges.

The reflected light beam from the surface 1 a passes through theobjective lens 20 again, and becomes a converging light beam directedtoward the detection unit 30. The spatial filter 31 is disposed betweenthe imaging lens 32 and the objective lens 20 at a position nearer tothe objective lens 20 than the image of the light source formed by theobjective lens 20. Thus, only the diffusely reflected component thatpasses through the spatial filter 31, is incident on the imaging lens32, and the image of the pattern impressed on the surface 1 a is formedon the imaging element 33 by the diffusely reflected component. Theimaging element 33 converts the image of the pattern into an electricsignal and outputs the signal to an image processing apparatus (notshown).

The parallel movement of the objective lens 20 is effective to preventthe effect of the ghosting light in the reflection-type system, similarto the eighth embodiment, in addition to compensate for the effect dueto the wedge-shaped object as described for the fifth embodiment.

In particular, when ghosting light, which is made by reflection at thesurface of the objective lens 20, is incident on the imaging lens 32 andoverlaps the position of the image pattern on the imaging element 33, itis difficult to read the image pattern because the contrast thereof islowered. In such a case, by adjusting the objective lens 20, by theparallel movement thereof, so that the ghosting light does not overlapthe image pattern, the contrast is not lowered and the pattern can becorrectly read. Further, if the surface 1 a is tilted, for example ifthe silicon wafer OR has a wedge shape, the position where the image ofthe light source is formed can be adjusted to coincide with the shadingregion of the spatial filter 31 by parallel movement of the objectivelens 20.

When the objective lens is moved to lower the effect of the ghostinglight, at least the surface of the objective lens 20, where the ghostinglight is made, must be a curved surface. When both the surfaces of theobjective lens 20 are curved as in the case of FIG. 26, both ghostinglight caused by the surface reflection arising at the lens surface onthe side of the light source and ghosting light due to the inner surfacereflection caused at the lens surface on the side of the silicon waferOR can be eliminated by the parallel movement of the objective lens 20.

FIG. 27 is a plan view showing the arrangement of the shift mechanism400 for parallel movement of the objective lens 20 and FIG. 28 is a sideview thereof. The objective lens 20 is supported by a flat-plate-shapedlens holder 410. The lens holder 410 is guided by a pair of guide rails420, 421 and is movable in a direction X. The guide rails 420, 421 arecoupled with each other by bridge members 430, 431 at ends thereof.Thus, a rectangular frame is formed by the guide rails 420,421 and thebridge members 430, 431.

A pair of tension springs 440, 441 are interposed between the lensholder 410 and the bridge member 430 such that the lens holder 410 isdrawn towards the bridge member 430. Further, a micrometer head 450 isfixed to the center of the bridge member 430 and an end of themicrometer head 450 abuts the lens holder 410 such that the position ofthe lens holder 410, that is, the position of the objective lens 20 maybe adjusted by rotating the micrometer head.

In particular, the micrometer head 450 may be rotated such that the lensholder 410 moves downward in the view of FIG. 27 against the urgingforce of the springs 440, 441, or such that the lens holder 410 movesupward in the view of FIG. 27 by being pulled by the springs 440, 441.Thus, the objective lens 20 can be set to an optimum position, that is,a position where the image of the light source coincides with theshading portion of the spatial filter 31 and ghosting light is notincident on the imaging element 33 by adjusting the micrometer head 450while observing an image formed on the imaging element 33.

If the silicon wafer OR also has a tilt or wedge shape in a directionperpendicular to the paper surface of FIG. 26 (direction Y), it ispreferable to also adjust the objective lens 20 in the direction Y. FIG.29 is a plan view showing an alternative shift mechanism by which theobjective lens 20 may also be adjusted in the direction Y, perpendicularto the optical axis Ax of the objective lens 20 as well as perpendicularto the direction X. The shift mechanism includes a Y-direction shiftmechanism 500 and the X-direction shift mechanism 400 shown in FIG. 27.

The Y-direction shift mechanism 500 includes a pair of guide rails 520,521 for guiding the X-direction shift mechanism 400 for parallelmovement and bridge members 530, 531 for coupling the guide rails 520,521 at the ends thereof to form a rectangular frame. A pair of tensionsprings 540, 541 are interposed between the bridge member 530 and theguide rail 421 of the x-direction shift mechanism 400 such that theX-direction shift mechanism 400 is drawn towards the bridge member 530.Further, the bridge member 530 is provided with a micrometer head 550,an end of which is abutted against the guide rail 421.

Similar to the above, the micrometer head 550 may be adjusted such thatthe x-direction shift mechanism 400 is moved against the urging force ofthe springs 540, 541 or such that the x-direction shift mechanism 400 ismoved by being pulled by the springs 540, 541. Thus, according to thearrangement of FIG. 29, if the silicon wafer OR has a tilt or wedgecomponent in any direction, the objective lens 20 can be set to aposition where the image of the light source coincides with the shadingportion of the spatial filter 31 and ghosting light is not incident onthe imaging element 33 by adjusting the objective lens 20 in the X-Ydirection.

Note that the amount of shift of the objective lens with respect to theangle of the silicon wafer OR is different depending upon a direction inwhich the angle is formed. For example, if the silicon wafer OR istilted 1 degree in the direction X, a light beam is angularly changedonly in the direction X and the amount of change is about 2 degrees,whereas if the silicon wafer OR is tilted 1 degree in the direction Y,the light beam is angularly changed 1.4 degrees in the direction Y andangularly changed by a small amount in the direction X. Therefore, whenthe silicon wafer OR is tilted in the direction X, it suffices to shiftthe objective lens 20 in only the direction X, however, when the siliconwafer OR is tilted in the direction Y, the objective lens 20 should beadjusted in both the directions X and Y.

Ninth Embodiment

FIG. 30(A) shows an optical system included in a pattern readingapparatus according to a ninth embodiment. The ninth embodiment is anexample of a filtering optical system for detecting a pattern containedin a light-transmission-type object.

A light beam emitted from a light source (not shown) passes through apinhole plate 12 to form a minute-area light source and is directlyincident on the object OT without passing through a lens. The light beampasses through the object OT, through an objective lens 22, and througha spatial filter 31, to form an image of the object OT on an imagingsurface 33 a. The spatial filter 31 has a shading region at the centerthereof for shading the portion of the light beam which forms the imageof the light source, i.e., a portion of the light beam that is notscattered by the object OT. The spatial filter 31 is disposed nearer tothe objective lens 22 than the paraxial image point IM of the lightsource.

In the optical systems for reading the pattern of thelight-transmission-type object in the above embodiments, an illuminatinglens is interposed between the pinhole plate 12 and the object OT, anobjective lens is interposed between the object OT and the spatialfilter, and an imaging lens is interposed between the spatial filter andthe imaging element. Thus, there are three lenses. On the other hand, inthe optical system shown in FIG. 30(A), since the light beam is directlyincident on the object OT, only two lenses are required, such that thecost of the optical system can be reduced.

The optical system in FIG. 30(B) shows a modification of the ninthembodiment in which the imaging lens 32 of FIG. 30(A) is not required.In this case, the objective lens 22 is provided with an imaging powerfor forming the image of a pattern on the imaging element 33. Accordingto the arrangement shown in FIG. 30(B), only one lens is included in theoptical system, such that the cost of the optical system can be furtherreduced from that of the arrangement in FIG. 30(A).

Tenth Embodiment

FIG. 31 shows an arrangement of a pattern reading apparatus according toa tenth embodiment. The tenth embodiment is an example in which theprinciple of reducing the number of lenses of the ninth embodiment isapplied to an optical system for detecting a pattern contained in alight-reflection-type object.

In an apparatus for reading a pattern on a light-reflection-type object,a single objective lens can act as both an illumination lens between alight source and an object and an objective lens between the object anda spatial filter, however, there are some problems. For example, when anincident light beam is obliquely incident on an object surface, anobjective lens with a large diameter is required. Further, when thelight beam is perpendicularly incident on the object surface, a beamsplitter is necessary to separate a reflected light beam and thequantity of light which reaches an imaging element is lowered to abouthalf that when the light beam is obliquely incident.

As shown in FIG. 31, the optical system of the apparatus includes anillumination unit 10, an objective lens 23 and a detection unit 30. Theillumination unit 10 and the detection unit 30 are positionedsymmetrically with respect to the normal of a surface 1 a so that when alight beam passing through the center of a pinhole which coincides withthe optical axis Ax1 of the illumination unit 10 is specularlyreflected, the light beam coincides with the optical axis Ax2 of thedetection unit 30.

The illumination unit 10 includes a lamp 11, a pinhole plate 12 with apinhole 12 a (a minute-area light source), and a diffusion plate 13. Thedetection unit 30 includes a spatial filter 31, an imaging lens 32, andan imaging element 33.

The light beam emitted from the light source is obliquely incident onthe surface 1 a as a divergent light beam and illuminates the surface 1a of a silicon wafer OR. The light beam is diffusely reflected by animpressed pattern on the surface 1 a and specularly reflected at otherportions. The reflected light beam passes through the objective lens 23as a converging light beam directed toward the detection unit 30. Theconverging light beam passes through the spatial filter 31 and animaging lens 32 and an image of the pattern on the surface 1 a is formedon the imaging element 33 by the diffusely reflected component. That is,the spatial filter 31 shades the specularly reflected component.

FIG. 32 shows a modification of the tenth embodiment in which theprinciple of the arrangement of FIG. 30(B) is applied to an opticalsystem for reading a light-reflection-type object. In particular, theoptical system shown in FIG. 32 does not include the imaging lens 32which is included in the optical system in FIG. 31. In this case, anobjective lens 23 is designed to form the image of the pattern on animaging element 33. Otherwise, the arrangement of the elements in thismodification is the same as the arrangement of the optical system inFIG. 31.

For this modification, an illumination light beam reaches the surface 1a of the silicon wafer OR as a divergent light beam, is reflected at thesurface 1 a, and is incident on the objective lens 23. The objectivelens 23 converges the reflected light beam and images the pattern on thesurface 1 a of the imaging element 33. The spatial filter 31 is disposednearer to the objective lens 23 than an image of the light source formedby the objective lens 23 and shades the specularly reflected componentof the reflected light beam. Therefore, the image of the pattern isformed on the imaging element 33 by the scatteringly reflected componentof the reflected light beam.

Eleventh Embodiment

FIG. 33 shows an optical system of a pattern reading apparatus accordingto an eleventh embodiment. The optical system includes an illuminationunit 10, an objective lens 23, and a detection unit 30. The illuminationunit 10 and the detection unit 30 are disposed symmetrically withrespect to a normal of a surface 1 a so that when a light beam passingthrough the center of a pinhole which coincides with the optical axisAx1 of the illumination unit 10 is specularly reflected, the light beamcoincides with the optical axis Ax2 of the detection unit 30.

The illumination unit 10 includes a lamp 11, a pinhole plate 12 with apinhole 12 a to form a minute-area light source, and a diffusion plate13. The illumination unit 10 is disposed such that an illumination lightbeam is obliquely incident on an object surface at a predeterminedincident angle. The detection unit 30 includes a spatial filter 31, animaging lens 32, and an imaging element 33. The spatial filter 31includes a shading region at the center thereof and is disposed nearerto the objective lens than a paraxial imaging point of the minute-arealight source.

An illumination light beam emitted from the illumination unit 10 isincident on the surface 1 a obliquely as a divergent light beam. At thesurface 1 a, the illumination light beam is diffusely reflected at animpressed pattern on the surface 1 a and specularly reflected atportions other than the pattern. The reflected light beam passes throughthe objective lens 23 and exits as a converging light beam directedtoward the detection unit 30. At the spatial filter 31, the scatteredreflected component passes through but the specularly reflectedcomponent does not. The scatterered reflected component passes throughthe imaging lens 32 to form an image of the pattern on the surface 1 aon the imaging element 33.

In this embodiment, the principal plane 32 a of the imaging lens 32, thesurface 1 a, and the imaging surface 33 a of the imaging element 33 aredisposed such that imaginary lines extending therefrom cross each otherat an axis RL, as shown by the dashed lines in FIG. 33, based onScheimpflug's rule. Such a disposition eliminates the effect of tilt ofthe image plane, which is conjugate to the surface 1 a, with respect tothe image surface 33 a. As a result, even if a pattern has a width in adirection parallel to a plane including both optical axes Ax1, Ax2, thepattern can be brought into focus as a whole.

The imaging element 33 converts the image of the pattern into anelectric signal and inputs the signal to an image processing apparatus(not shown). The image processing apparatus displays the image of thepattern on a display screen or analyzes the content of the pattern usinga character recognition algorithm or the like.

Note that, when the surface 1 a is not parallel with the image surface33 a, as in this embodiment, since a magnification changes dependingupon position in a direction parallel to the plane including both theoptical axes, a formed pattern is distorted. If the distortion of theimage of the pattern affects reading, the distortion can be compensatedfor by image processing such as an affine transformation or the like.

FIG. 34 shows a modification of the optical system of the eleventhembodiment. The optical system in FIG. 34 does not include the imaginglens 32 of FIG. 33 and an objective lens 23 is designed to form an imageof a pattern directly on an imaging element 33. In particular, in thiscase, the principal plane 23 a of the objective lens 23, a surface 1 a,and the image surface 33 a of the imaging element 33 are disposed suchthat imaginary lines extending therefrom cross each other on an axis RL,as shown by the dashed lines in FIG. 34, based on Scheimpflug's rule.Otherwise, the arrangement is the same as that of the optical system ofFIG. 33.

In this case, the illumination light beam emitted from the illuminationunit 10 is incident on the surface 1 a of the silicon wafer OR as adivergent light beam and is reflected at the surface 1 a. The objectivelens 23 converges the reflected light beam and images the pattern on thesurface 33 a of the imaging element 33. The spatial filter 31 isdisposed nearer to the objective lens 23 than the paraxial imaging pointof the minute-area light source and shades the specularly reflectedcomponent. Thus, the diffusely reflected light beam passes through thespatial filter 31 and forms the image of the pattern on the imagingelement 33.

According to the eleventh embodiment, an object surface can be madeconjugate with an imaging surface by disposing the lens having theimaging function and the imaging surface according to Scheimpflug'srule, such that an in-focus pattern image can be obtained.

Twelfth Embodiment

FIG. 35 shows an optical system for a pattern reading apparatusaccording to a twelfth embodiment. The optical system includes a lightemitting diode 10 a, an objective lens 20, an imaging lens 32, and animaging element 33. The light emitting diode 10 a (light source) isdisposed at a position which is conjugate with a center of curvature ofa surface 1 a of a silicon wafer OR through the objective lens 20. Theimaging lens 32 is disposed at a position which is farther from thesilicon wafer OR than the light emitting diode 10 a. Further, opticalaxes Ax of the objective lens 20 and the imaging lens 32 are coincidentand perpendicular to the surface 1 a. Because the surface 1 a is flat,in the example shown in FIG. 35, the light emitting diode 10 a ispositioned approximately at a focal point of the objective lens 20.

In the twelfth embodiment, the light beam emitted from the lightemitting diode 10 a passes through the objective lens 20 and illuminatesthe surface 1 a of the silicon wafer OR as a parallel light beamperpendicular to the surface 1 a. The illumination light beam isdiffusely reflected at an impressed pattern on the surface 1 a andspecularly reflected at other portions. The specularly reflectedcomponent is converged to the position of the light emitting diode 10 aas the reflected light beam passes through the objective lens 20 and isshaded by the light emitting diode 10 a.

The diffusely reflected component is not shaded by the light emittingdiode 10 a and passes through the imaging lens 32 to form an image ofthe pattern on the imaging element 33.

FIG. 36 shows a modification of the optical system according to thetwelfth embodiment. As shown in FIG. 36 a light source includes a lightemitting element, such as a semiconductor laser 11 a, a light guidefiber 14, and a coupling lens 15. The semiconductor laser 11 a and thecoupling lens 15 are disposed outside of the optical path between thesurface 1 a and the imaging element 33. The light guide fiber 14 extendsfrom an entrance end 14 a near the coupling lens 15 to an exit end 14 bdisposed at a position which is conjugate with the center of curvatureof the surface 1 a through the objective lens 20. Since, in FIG. 36, thesurface 1 a is flat, the exit end 14 b of the light guide fiber 14 isdisposed approximately at the focal point of the objective lens 20.Otherwise, the arrangement of the optical system is the same as that ofFIG. 35.

With this modification, the laser beam emitted from the semiconductorlaser 11 a is incident on the entrance end 14 a of the light guide fiber14 through the coupling lens 15. Then, the laser beam emitted from theexit end 14 b of the light guide fiber 14 illuminates the surface 1 athrough the objective lens 20 as a parallel light beam. Since thereflected light beam from the surface 1 a is converged when it passesthrough the objective lens 20 again, a specularly reflected component isshaded by the end of the fiber and only a diffusely reflected componentpasses through the imaging lens 32 to form an image of a pattern on thesurface 1 a on the imaging element 33.

According to the twelfth embodiment, the size of the optical system canbe reduced as compared with an optical system in which the light beam isobliquely incident. Further, the quantity of light is larger than when abeam splitter is used.

Still further, the light source acts as a spatial filter for shading thespecularly reflected component of light from the object surface so thata distinct image of the pattern can be formed by a diffusely reflectedcomponent without the provision of an additional filter.

The present disclosure relates to subject matter contained in JapanesePatent Applications No. HEI 08-241112, filed on Aug. 23, 1996, No. HEI08-301076, filed on Oct. 25, 1996, No. HEI 08-342775, filed on Dec. 6,1996, No. HEI 08-342776, filed on Dec. 6, 1996, No. HEI 08-342777, filedon Dec. 6, 1996, No. HEI 08-342778, filed on Dec. 6, 1996, No. HEI09-65333, filed on Mar. 4, 1997, No. HEI 09-74497, filed on Mar. 11,1997, No. HEI 09-65334, filed on Mar. 4, 1997, No. HEI 09-134312, filedon May 8, 1997, and No. HEI 09-165422, filed on Jun. 6, 1997, which areexpressly incorporated herein by reference in their entirety.

What is claimed is:
 1. A pattern reading apparatus, comprising: aminute-area light source; an objective lens that makes an illuminationlight beam from said light source incident on a reflection surfacehaving a pattern formed thereon as an object to be read and convergesthe light beam reflected from the reflection surface; an imaging lensfor forming, at a predetermined position, an image of the pattern from adiffusely reflected component of the reflected light beam, which haspassed through said objective lens; an imaging element disposed at thepredetermined position where the image of the pattern is formed, forreading the pattern; and a spatial filter disposed in an optical pathbetween said imaging lens and said objective lens so that said spatialfilter is approximately conjugate with said light source through saidobjective lens, said spatial filter having a shading portion that shadesthe light beam in a range that is conjugate with at least said lightsource, said spatial filter shielding a specularly reflected componentof light and forming an image on the imaging element by the diffuselyreflected component of illumination light, wherein said light source isoptically conjugate with a center of curvature of the reflective surfaceof the object to be read through said objective lens.
 2. The patternreading apparatus according to claim 1, wherein the object to be read isa plane and said objective lens causes the illumination light beam to beincident on the object to be read as a parallel light beam.
 3. Thepattern reading apparatus according to claim 1, wherein the object to beread is a convex spherical surface and said objective lens causes theillumination light beam to be incident on the object to be read as aconverging light beam.
 4. The pattern reading apparatus according toclaim 1, wherein the object to be read which is a concave sphericalsurface and said objective lens causes the illumination light beam to beincident on the object to be read as a diverging light beam.
 5. Thepattern reading apparatus according to claim 1, wherein said lightsource is disposed at a position where the illumination light beam iscaused to be obliquely incident on the reflection surface.
 6. Thepattern reading apparatus according to claim 5, wherein said objectivelens is composed of a first lens through which the illumination lightbeam passes and a second lens through which the reflected light beampasses and said first and second lenses are disposed such that theoptical axes thereof cross each other on the side of the reflectionsurface.
 7. A pattern reading apparatus according to claim 1, whereinsaid objective lens is disposed such that an optical axis thereof isperpendicular to the reflection surface and said light source and saidimaging element are disposed opposite each other on different sides ofthe optical axis.
 8. The pattern reading apparatus according to claim 1,wherein said light source is disposed at a position where theillumination light beam is perpendicularly incident on the reflectionsurface and an optical path splitting element is disposed in the opticalpath between said light source and said objective lens to separate theoptical path of the illumination light beam emitted from said lightsource from the optical path of the reflected light beam from thereflection surface.